[Ord. No. 223, 4/12/2023]
A. 
Applicants proposing regulated activities in the municipality which are not exempt under section 20-106 shall submit a stormwater management site plan (SWM site plan) to the municipality for review and approval in accordance with parts 3 and 4. SWM site plans approved by the municipality shall be on site throughout the duration of the regulated activity.
B. 
The stormwater management and runoff control criteria and standards in this chapter shall apply to the total proposed regulated activity, even if it is to take place in stages. The measurement of impervious surfaces shall include all of the impervious surfaces in the total proposed regulated activity even if the development is to take place in stages.
C. 
No regulated activity within the municipality shall commence until:
(1) 
The municipality issues approval of a SWM site plan, which demonstrates compliance with the requirements of this chapter; and
(2) 
The applicant has received a letter of adequacy or approval for the erosion and sediment control plan review by the municipality and the conservation district (if required), and has received all other local, state and federal permit approvals required for the project involving the regulated activity.
D. 
Neither submission of a SWM site plan under the provisions herein nor compliance with the provisions of this chapter shall relieve any person from responsibility for damage to any person or property otherwise imposed by law.
E. 
The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in section 20-304 for the overall site design and for selection, location, and design of features and BMPs to be used to comply with the requirements of this chapter.
F. 
To the maximum extent practicable, post-construction stormwater shall be discharged within the drainage area of the same stream or water body receiving the runoff prior to construction of the proposed regulated activity.
G. 
For regulated activities with one acre or more of proposed earth disturbance, existing drainage peak rate discharges up to and including the 100-year storm and the volume of runoff up to and including the two-year storm onto or through adjacent property(ies) or downgradient property(ies), including diffuse drainage discharge, shall not be altered in any manner by regulated activities under this chapter without written permission from, and, where applicable as determined by the municipality, an easement and agreement with the affected landowner(s) for conveyance of discharges onto or through their property. Altered stormwater discharges shall be subject to any applicable discharge criteria specified in this chapter.
H. 
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in sections 20-305, 20-306, 20-307 and 20-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
I. 
If site conditions preclude capture of runoff from limited portions of the disturbed area for achieving water quality volume control standards, stream channel protection standards, and the two-year, storm event peak runoff rate reduction standards for new development required by this chapter, the applicant shall propose alternate methods to mitigate the bypass of the BMPs, subject to the approval of the township. In no case shall resulting peak rate be greater than the predevelopment peak rate for the equivalent design storm.
J. 
For all regulated activities, erosion, and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated activities (i.e., during construction) as required to meet the purposes and requirements of this chapter, to meet the erosion and sediment control requirements of the municipality, if applicable, and to meet all requirements under Title 25 of the PA Code and the Clean Streams Law.
K. 
For all regulated activities, permanent BMPs and conveyances shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.
L. 
The design of all BMPs and conveyances shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems as identified by the municipality. The municipality reserves the right to disapprove any design that would result in construction in an area affected by existing stormwater problem(s) or continuation of an existing stormwater problem(s).
M. 
Existing wetlands, either on the site or on an adjacent property, shall not be used to meet the minimum design requirements for stormwater management or stormwater runoff quality treatment. Stormwater discharges to existing wetlands shall not degrade the quality or hydrologic integrity of the wetland.
N. 
Hotspots runoff controls. Specific structural or pollution prevention practices may be required, as determined to be necessary by the township, to pretreat runoff from hotspots prior to infiltration. Following is a list of examples of hotspots:
(1) 
Vehicle salvage yards and recycling facilities;
(2) 
Vehicle fueling stations;
(3) 
Vehicle service and maintenance facilities;
(4) 
Vehicle and equipment cleaning facilities;
(5) 
Fleet storage areas (bus, truck, etc.);
(6) 
Industrial sites based on standard industrial classification codes;
(7) 
Marinas (service and maintenance areas);
(8) 
Outdoor liquid container storage;
(9) 
Outdoor loading/unloading facilities;
(10) 
Public works storage areas;
(11) 
Facilities that generate or store hazardous materials;
(12) 
Commercial container nursery;
(13) 
Contaminated sites/brownfields;
(14) 
Other land uses and activities as designated by the municipality.
O. 
Contaminated and brownfield sites. Where BMPs may contribute to the migration of contaminants in groundwater, the water quality and runoff volume, stream channel protection, and peak rate control standards shall be met; however, at the township's discretion, the minimum infiltration requirement may be reduced or eliminated commensurate with the contaminated area and the required water quality and runoff control measures may be increased to mitigate the reduced infiltration requirement for the contaminated area.
P. 
Applicants shall utilize the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual), as amended, or other sources acceptable to the township, for testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies.
Q. 
For areas underlain by karst or carbonate geology that may be susceptible to the formation of sinkholes and other karst features, the location, type, and design of infiltration BMPs shall be based on a site evaluation conducted by a qualified licensed professional and based on the PA BMP Manual (as amended) or other design guidance acceptable to the township.
[Ord. No. 223, 4/12/2023]
1. 
The following permit or other regulatory requirements may apply to certain regulated activities and shall be met prior to (or as a condition of) final approval by the municipality of the SWM site plan and prior to commencement of any regulated activities, as applicable:
A. 
All regulated activities subject to permit or regulatory requirements by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102, or erosion and sediment control requirements of the municipality.
B. 
Work within natural drainage ways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
C. 
Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
D. 
Any BMP or conveyance that would be located on or discharge to a state highway right-of-way or require access to or from a state highway and be subject to approval by PennDOT.
E. 
Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
[Ord. No. 223, 4/12/2023]
A. 
No regulated activity within the municipality shall commence until:
(1) 
The municipality receives documentation that the applicant has received:
(a) 
A "letter of adequacy" from the conservation district or other approval from PADEP in compliance with Title 25 Chapter 102 of the Pennsylvania Code of an erosion and sediment control plan for construction activities for projects where the area of disturbance exceeds one acre, where pond dredging is involved, or when the disturbance is associated with activities described under Title 25 Chapter 105 of the Pennsylvania Code permits.
(b) 
A PADEP NPDES permit for stormwater discharges associated with construction activities as required under Title 25 Pennsylvania Code Chapter 92.a, if applicable.
(c) 
Evidence of any other permit(s) or approvals required for the regulated activities.
(2) 
An erosion and sediment control plan has been approved by the municipality, if required.
B. 
A copy of the erosion and sediment control plan and any required permit(s), as required by PADEP regulations, shall be available on the site at all times.
C. 
Additional erosion and sediment control design standards found in section 20-311 shall apply where required by table 20-106.1.
[Ord. No. 223, 4/12/2023]
1. 
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. For regulated activities with 10,000 or more square feet of proposed earth disturbance or 2,000 or more square feet of regulated impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in subsection 20-402.C) that the design sequence, objectives, and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
A. 
First, identify and delineate all existing natural resources and natural and manmade hydrologic features listed in subsection 20-402.B.(8) that are located within the site, or receive discharge from, or may be impacted by the proposed regulated activity.
B. 
Second, provide a prioritized listing of these resources and features to identify:
(1) 
Those to be incorporated into the site design in a manner that provides protection from any disturbance or impact from the proposed regulated activity;
(2) 
Those to be protected from further disturbance or impact but for which the proposed regulated activity will provide improvement to existing conditions;
(3) 
Those that can be incorporated into and utilized as components of the overall site design in a manner that protects or improves their existing conditions while utilizing their hydrologic function within the limits of their available capacity (e.g., for infiltration, evapotranspiration, or reducing pollutant loads, runoff volume or peak discharge rates, etc.) to reduce the need for or size of constructed BMPs; and
(4) 
Those that may be considered for alteration, disturbance, or removal.
C. 
Third, develop the site design to achieve the following:
(1) 
Recognize and incorporate the priorities identified in subsection 20-304.B as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2) 
Minimize earth disturbance (both surface and subsurface);
(3) 
Maximize protection of or improvement to natural resources and special management areas;
(4) 
Minimize the disturbance of natural site hydrology, in particular natural drainage features and patterns, discharge points and flow characteristics, natural infiltration patterns and characteristics, and natural channel and floodplain conveyance capacity;
(5) 
Incorporate natural hydrologic features and functions identified in subsection 20-304.B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6) 
Maximize infiltration and the use of natural site infiltration features, patterns and conditions, and evapotranspiration features;
(7) 
Apply selective grading design methods to provide final grading patterns or preserve existing topography in order to evenly distribute runoff and minimize concentrated flows;
(8) 
Minimize the cumulative area to be covered by impervious surfaces and:
(a) 
Minimize the size of individual impervious surfaces;
(b) 
Separate large impervious surfaces into smaller components;
(c) 
Disconnect runoff from one impervious surface to another; and
(d) 
Utilize porous materials in place of impervious wherever practicable;
(9) 
Minimize the volume and peak discharge rates of stormwater generated;
(10) 
Avoid or minimize stormwater runoff pollutant loads and receiving stream channel erosion;
(11) 
Locate infiltration and other BMPs:
(a) 
At or as near to the source of generation as possible; and
(b) 
At depths that are as shallow as possible;
(12) 
Prioritize the selection and design of BMPs as follows:
(a) 
Nonstructural and vegetation BMPs, then
(b) 
Structural (surface and subsurface) BMPs;
(13) 
For flow volumes requiring conveyance from the source of generation to a BMP for management, give preference to open channel conveyance techniques that provide infiltration and water quality benefits, and landscaped-based management in common open space areas, where practicable; and
(14) 
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of subsections 20-304.B and 20-304.C. Appendix 20-B presents additional discussion of "conservation design" and "low impact development."
D. 
The procedures set forth above shall be utilized to the maximum extent practicable for the overall site design and selection, location, and design of features and BMPs to be used to comply with the requirements of sections 20-305, 20-306, 20-307 and 20-308.
[Ord. No. 223, 4/12/2023]
1. 
To control post-construction stormwater impacts from regulated activities and meet state water quality requirements, BMPs shall be provided in the site design that replicate predevelopment stormwater infiltration and runoff conditions, such that post-construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. The green infrastructure and low impact development (LID) practices provided in the PA BMP Manual, as well as the guidance on green infrastructure, LID and conservation design (CD) provided in appendix 20-B, shall be utilized for all regulated activities wherever possible. The applicant shall comply with the following water quality and runoff volume requirements for all regulated activities, including all new development and redevelopment activities:
A. 
The post-construction total runoff volume shall not exceed the predevelopment total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation (design storm) or a minimum of 1 1/2 inches of runoff from all regulated impervious surfaces shall be managed. Whichever volume is greater shall be managed. The water quality and runoff volume to be managed shall consist of any runoff volume generated by the proposed regulated activity over and above the predevelopment total runoff volume and shall be captured and permanently retained or infiltrated on the site. Permanent retention options may include, but are not limited to, reuse, evaporation, transpiration, and infiltration.
B. 
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in subsection 20-309.D.
C. 
The design of the stormwater management facility outlet shall provide for protection from clogging and unwanted sedimentation.
D. 
BMPs that moderate the temperature of stormwater shall be used to protect the temperature of receiving waters.
E. 
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of section 20-306. The infiltration volume required under section 20-306 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F. 
Runoff from the disturbed area shall be treated for water quality prior to entering existing waterways or water bodies. If a stormwater management practice does not provide water quality treatment, then water quality BMPs shall be utilized to provide pre-treatment prior to the runoff entering the stormwater management practice.
G. 
The municipality may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in subsection 20-301.N.
H. 
When the regulated activity contains or is divided by multiple drainage areas, the water quality and runoff volume shall be separately addressed for each drainage area.
I. 
Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations.
J. 
Areas located outside of the site (i.e., areas outside of the regulated activity) may be excluded from the calculation of the water quality and runoff volume requirements.
K. 
Water quality and volume control practices shall be selected and designed to meet the criteria of subsection 20-304.C that apply to water quality and volume control.
L. 
Evapotranspiration may be quantified and credited towards meeting volume requirements according to the PADEP Post Construction Stormwater Management (PCSM) Spreadsheet and Instructions (December 2020) or the most recent guidance from PADEP.
[Ord. No. 223, 4/12/2023]
1. 
Providing for infiltration consistent with the natural hydro logic regime is required to compensate for the reduction in the recharge that occurs when the ground surface is disturbed, or impervious surface is created or expanded. The applicant shall achieve the following infiltration requirements:
A. 
For regulated activities involving both new development and redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required in section 20-305. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PA DEP design guidance. If the runoff volume required by section 20-305 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PA DEP guidance, such as the managed release concept, may be used to manage this volume with approval from the township.
B. 
For regulated activities involving both new development and redevelopment, the volume of a minimum of one inch of runoff from all regulated-impervious surfaces shall be infiltrated.
C. 
If the requirements of subsection 20-306.A or 20-306.B cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the township why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated.
D. 
Only if a minimum infiltration of the first 1/2 inch of runoff volume cannot be physically accomplished on the site, shall a waiver from section 20-306 be considered by the municipality, in accordance with section 20-111.
E. 
If site conditions preclude capture of runoff from portions of the impervious surfaces, the infiltration volume for the remaining area shall be increased an equivalent amount to offset the loss.
F. 
When a project contains or is divided by multiple watersheds, the infiltration volume shall be separately addressed for each watershed.
G. 
Existing impervious surfaces located in areas outside of the site (i.e., outside of the regulated activity) may be excluded from the calculation of the required infiltration volume.
H. 
A detailed soils evaluation of the site shall be conducted by a qualified professional and at a minimum shall address soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be conducted by a qualified licensed professional and shall be consistent with the PA BMP Manual (as amended) (or other guidance acceptable to the township) and in general shall:
(1) 
Analyze hydrologic soil groups as well as natural and manmade features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of sub-grade stability; infiltration may not be ruled out without conducting these tests.
(2) 
Provide field tests such as double ring infiltrometer or other hydraulic conductivity tests (at the elevation of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Standard septic/sewage percolation tests are not acceptable for design purposes.
(3) 
Design the infiltration facility for the required retention (infiltration) volume based on field-determined infiltration capacity (and apply safety factor as per applicable design guidelines) at the elevation of the proposed infiltration surface.
(4) 
On-lot infiltration features are encouraged; however, it shall be demonstrated to the township that the soils are conducive to infiltration on the identified lots.
I. 
Infiltration BMPs shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(1) 
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by karst or carbonate geology (see subsection 20-306.M).
(2) 
An infiltration rate sufficient to accept the additional stormwater volume and drain completely as determined by field tests conducted by the applicant.
(3) 
The infiltration facility shall completely drain the retention (infiltration) volume within three days (72 hours) from the end of the design storm.
J. 
All infiltration practices shall:
(1) 
Be selected and designed to meet the criteria of subsection 20-304.C that are applicable to infiltration;
(2) 
Be set back at least 15 feet from all buildings and features with sub-grade elements (e.g., basements, foundation walls, etc.), unless otherwise approved by the township;
(3) 
For any infiltration practice that collects runoff from shared or multiple features and that is located within 50 feet of a building or feature with sub-grade elements (e.g., basements, foundation walls, etc.), the bottom elevation shall be set below the elevation of the sub-grade element.
K. 
Infiltration facilities shall, to the maximum extent practicable, be located to avoid introducing contaminants to groundwater:
(1) 
When a hotspot is located in the area draining to a proposed infiltration facility, an evaluation of the potential of groundwater contamination from the proposed infiltration facility shall be performed, including a hydrogeologic investigation (if necessary) by a qualified licensed professional to determine what, if any, pretreatment, or additional design considerations are needed to protect ground water quality.
(2) 
When located within a "well head protection area" of a public water supply well, infiltration practices shall be in conformance with the applicable approved source water protection assessment or source water protection plan.
(3) 
The applicant shall provide appropriate safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
L. 
During site construction, all infiltration practice components shall be protected from compaction due to heavy equipment operation or storage of fill or construction material. Infiltration areas shall also be protected from sedimentation. Areas that are accidentally compacted or graded shall be remediated to restore soil composition and porosity. Adequate documentation to this effect shall be submitted to the township for review. All areas designated for infiltration shall not receive runoff until the contributory drainage area has achieved final stabilization.
M. 
Consideration of infiltration BMPs for areas underlain by karst or carbonate geology is encouraged, but only where the design, supporting calculations, results of soils or other site investigations or other documentation are provided to the municipality demonstrating that the potential or likelihood of subsidence or sinkholes is minimal. Evaluation of site conditions and infiltration design shall rely on guidance in the PA BMP Manual (as amended) or other guidance acceptable to the township.
N. 
Groundwater quality of the carbonate aquifer shall be protected from infiltration of pollutants. At a minimum, stormwater runoff from hotspots (i.e., sources of significant pollutant runoff) shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration. Where soil characteristics are insufficient to provide removal of pollutants from sources other than hotspots, stormwater runoff shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration.
O. 
Where sediment transport in the stormwater runoff is anticipated to reach the infiltration system, appropriate permanent measures to prevent or collect sediment shall be installed prior to discharge to the infiltration system.
P. 
Where roof drains are designed to discharge to infiltration practices, they shall have appropriate measures to prevent clogging by unwanted debris (for example, silt, leaves and vegetation). Such measures shall include, but are not limited to, leaf traps, gutter guards, and cleanouts.
Q. 
All infiltration practices shall have appropriate positive overflow controls.
R. 
No sand, salt or other particulate matter may be applied to a porous surface material for winter ice conditions.
S. 
The following procedures and materials shall be required during the construction of all subsurface facilities:
(1) 
Excavation for the infiltration facility shall be performed with equipment that will not compact the bottom of the seepage bed/trench or like facility.
(2) 
The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.
(3) 
Only clean aggregate with documented porosity, free of fines, shall be allowed.
(4) 
The tops, bottoms and sides of all seepage beds, trenches, or like facilities shall be covered with drainage fabric. Fabric shall be non-woven fabric acceptable to the township.
(5) 
Stormwater shall be distributed throughout the entire seepage bed/trench or like facility and provisions for the collection of debris shall be provided in all facilities.
[Ord. No. 223, 4/12/2023]
1. 
For regulated activities involving new development with one or more acres of earth disturbance, the applicant shall comply with the following stream channel protection requirements to minimize stream channel erosion and associated water quality impacts to the receiving waters:
A. 
The peak flow rate of the post-construction two-year, twenty-four-hour design storm shall be reduced to the predevelopment peak flow rate of the one-year, twenty-four-hour duration precipitation, using the SCS Type II distribution.
B. 
To the maximum extent practicable, and unless otherwise approved by the township, the post-construction one-year, twenty-four-hour storm flow shall be detained for a minimum of 24 hours and a maximum not to exceed 72 hours from a point in time when the maximum volume of water from the one-year, twenty-four-hour storm is stored in a proposed BMP (i.e., when the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the orifice is at the invert of the proposed BMP).
C. 
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in subsection 20-309.D.
D. 
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter unless otherwise approved by the township, and a trash rack shall be installed to prevent clogging. For sites with small drainage areas contributing to the BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition.
E. 
When the calculated orifice size is below three inches, gravel filters (or other methods) are recommended to discharge low-flow rates subject to the township's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function.
F. 
All proposed stormwater management facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.
G. 
When a regulated activity contains or is divided by multiple drainage areas, the peak flow rate control shall be separately addressed for each drainage area.
[Ord. No. 223, 4/12/2023]
1. 
The applicant shall comply with the following peak flow rate control requirements for all regulated activities including those that involve new development and redevelopment.
A. 
Post-construction peak flow rates from any regulated activity shall not exceed the predevelopment peak flow rates as shown for each of the design storms specified in table 20-308.1.
TABLE 20-308.1
Peak Rate Control Standards
(Peak Flow Rate of the Post-construction Design Storm Shall be Reduced to the Peak Flow Rate of the Corresponding Predevelopment Design Storm Shown in the Table)
POST-CONSTRUCTION DESIGN STORM FREQUENCY
(24-Hour Duration)
PREDEVELOPMENT DESIGN STORM
New Development Regulated Activities
Redevelopment Regulated Activities
2-Year
1-Year
2-Year
5-Year
5-Year
5-Year
10-Year
10-Year
10-Year
25-Year
25-Year
25-Year
50-Year
50-Year
50-Year
100-Year
100-Year
100-Year
B. 
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in subsection 20-309.D.
C. 
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total regulated impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in table 20-308.1, using the redevelopment ground cover assumptions presented in subsection 20-309.D. This design criterion for redevelopment is only permitted with approval of township. It shall result in no impact on downstream properties.
D. 
Only the area of the proposed regulated activity shall be subject to the peak flow rate control standards of this chapter. Undisturbed areas for which the discharge point has not changed are not subject to the peak flow rate control standards.
E. 
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to peak flow rate control requirements. Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
F. 
When a regulated activity contains or is divided by multiple drainage areas, the peak flow rate controls shall be separately addressed for each drainage area.
G. 
The effect of structural and non-structural stormwater management practices implemented as part of the overall site design may be taken into consideration when calculating total storage volume and peak flow rates.
[Ord. No. 223, 4/12/2023]
A. 
Stormwater runoff from all regulated activity sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique(s) that is based on the NRCS soil cover complex method. Table 20-309.1 summarizes acceptable computation methods. The method selected for use shall be based on the individual limitations and suitability of each method for a particular site. The use of the rational method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval by the township.
TABLE 20-309.1
ACCEPTABLE COMPUTATION METHODOLOGIES FOR SWM SITE PLAN
METHOD
DEVELOPED BY
APPLICABILITY
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary.
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans where limitations described in TR-55 are met.
HEC-1/HEC-HMS
U.S. Army Corps of Engineers
Applicable where use of a full hydrologic computer model is desirable or necessary.
Rational method (or commercial computer package based on rational method)
Emil Kuichling (1889)
For sites up to five acres, or as approved by the municipality.
Other methods
Varies
Other computation methodologies approved by the municipality.
B. 
All calculations using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms consistent with this chapter. Rainfall depths used shall be obtained from NOAA Atlas 14 values consistent with a partial duration series. When stormwater calculations are performed for routing procedures or infiltration, water quality and runoff volume functions, the duration of rainfall shall be 24 hours.
C. 
All calculations using the rational method shall use rainfall intensities consistent with appropriate times-of-concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584). Times of concentration shall be calculated based on the methodology recommended in the respective model used. Times of concentration for channel and pipe flow shall be computed using Manning's equation.
D. 
The applicant shall utilize the following ground cover assumptions for all predevelopment water quality and runoff volume, infiltration volume and peak flow rate calculations:
(1) 
For regulated activities involving new development, the following ground cover assumptions shall be used:
(a) 
For areas that are woods (as defined in part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(b) 
For all other areas (including all impervious surfaces), predevelopment calculations shall assume ground cover of "meadow."
(2) 
For regulated activities involving redevelopment, the following ground cover assumptions shall be used:
(a) 
For areas that are woods (as defined in part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(b) 
For areas that are not woods or not impervious surfaces, predevelopment calculations shall assume ground cover of "meadow."
(c) 
For areas that are impervious surfaces, predevelopment calculations shall assume at least 20% of the existing impervious surface area to be disturbed as "meadow" ground cover.
(3) 
The applicant shall determine which stormwater standards apply to the proposed regulated activity as follows:
(a) 
Stormwater standards for new development shall apply to all proposed regulated activities that involve only new development activities as defined in this chapter.
(b) 
Stormwater standards for redevelopment shall apply to all proposed regulated activities that involve only redevelopment activities as defined in this chapter.
(c) 
At the discretion of the township, regulated activities that involve a combination of both new development and redevelopment activities, as defined in this chapter, may either:
1) 
Apply the stormwater standards (redevelopment or new development) that are associated with the activity that involves the greatest amount of land area; or
2) 
Apply the redevelopment and new development stormwater standards to the corresponding redevelopment and new development portions of the proposed regulated activity.
E. 
Runoff curve numbers (CN) for both predevelopment and proposed (postconstruction) conditions to be used in the soil cover complex method shall be obtained from table C-1 in appendix 20-C of this chapter.
F. 
Runoff coefficients (C) for both predevelopment and proposed (post-construction) conditions for use in the rational method shall be obtained from table C-2 in appendix 20-C of this chapter.
G. 
Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations.
H. 
Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for Manning's roughness coefficient (n) shall be consistent with table C-3 in appendix 20-C of this chapter.
I. 
Runoff calculations shall include the following assumptions:
(1) 
Average antecedent moisture conditions (for the soil cover complex method only for example, TR-55, TR-20).
(2) 
A type II distribution storm (for the soil cover complex method only for example, TR-55, TR-20).
[Ord. No. 223, 4/12/2023]
1. 
Conveyance systems (gutters, swales, open vegetated channels, drainageways, and storm sewers).
A. 
Design Standards. The following design standards shall apply:
(1) 
Road gutters or swales shall be designed to have a maximum flow depth of six inches if no stormwater is conveyed in the cartway of the street. The maximum flow depth permitted across a street at the intersection of two streets is one inch. All curbed streets shall be designed with a maximum flow depth of two inches.
(2) 
Storm sewer pipes, culverts, manholes, inlets, endwalls and end sections proposed for dedication or located along streets shall conform to the requirements of the Pennsylvania Department of Transportation, Bureau of Design, Standards for Roadway Construction, Publication No. 72, in effect at the time the design is submitted, as modified by the adopted township construction standards.
(3) 
The capacities of the pipes, gutters, inlets, culverts, outlet structures, swales and channels shall consider all possible hydraulic conditions. The design standards shall be in accordance with Pennsylvania Department of Transportation Publication No. 13, Design Manual, No. 2, as amended.
(4) 
The capacities of open channels shall be calculated using the Manning equation. Permissible open channel velocities and design standards shall be in accordance with good engineering practice as documented in the Engineering Field Manual for Conservation Practices, USDA, SCS, or in "Design Charts for Open-Channel Flow," Hydraulic Design Series No. 3, United States Department of Transportation.
(5) 
Flood relief channels shall be provided and designed to convey the runoff from the 100-year, twenty-four-hour storm, such that a positive discharge of the runoff to an adequate receiving stream or other conveyance system occurs without allowing the runoff to encroach upon other properties.
(6) 
All open channels shall be design with a 1/2 foot freeboard above the design flow elevation.
(7) 
Endwalls shall be used where stormwater runoff enters or leaves the storm sewer. Endwalls shall be fitted with self-cleaning trash racks that will prevent pets and small children from entering the pipes.
(8) 
Inlets shall be placed on both sides of the street at low spots, as needed, to collect runoff from road gutters, at abrupt changes in horizontal or vertical alignment of storm sewers, and at a maximum distance of 300 feet apart along a storm sewer pipe. Inlets shall be depressed two inches below the grade of the gutter or road surface. Manholes may be substituted for inlets at locations where inlets are not required to handle surface water runoff.
(9) 
All storm sewer pipes shall be laid to a minimum depth of one foot from subgrade to the crown of the pipe, two feet within the street cartway.
(10) 
All storm sewers greater than 15 inches in diameter shall be fitted with appropriately designed trash racks at both the inflow and discharge ends.
(11) 
Flow velocities from any storm sewer may not result in a deflection of the receiving channel. Energy dissipaters shall be placed at the outlets of all storm sewer pipes where flow velocities exceed maximum permitted channel velocities.
(12) 
Minimum sizes.
(a) 
Curved pipes, tee joints, elbows and wyes are prohibited for except for all pipes with at least a thirty-six-inch diameter.
(b) 
Storm sewer pipes and culverts, other than those used for detention basin outlets, shall have a minimum diameter of 15 inches.
(13) 
All trenches for pipe installed beneath the cartway and within three feet of an inlet or manhole shall be backfilled with 2A modified stone or other stone approved by the township engineer.
B. 
Material Standards. The following material standards shall apply: storm sewer pipes and culverts, other than those used for detention basin outlets, shall be made of reinforced concrete or corrugated smooth-walled high density polyethylene.
C. 
Field Construction Standards. The following construction standards shall apply:
(1) 
All storm sewer pipes, culverts, manholes, inlets, endwalls and end sections shall be installed in accordance with the Pennsylvania Department of Transportation, Publication 408, as amended.
(2) 
Storm sewer pipes and culverts, other than those used for detention basin outlets, shall be installed with sufficient slope to provide a minimum velocity of three feet per second when flowing full.
[Ord. No. 223, 4/12/2023]
1. 
All activities involving earth disturbance shall be consistent with the following performance standards and those of the Pennsylvania Department of Environmental Protection:
A. 
All earth disturbance activities shall be conducted in such a way as to prevent accelerated erosion and resulting sedimentation. To accomplish this, all persons engaged in earth disturbance activities shall design, implement, and maintain erosion and sedimentation control measures consistent with the Pennsylvania Department of Environmental Protection Erosion and Sediment Pollution Control Program Manual (March 2012, or current version).
B. 
In order to prevent accelerated erosion and resulting sedimentation, earth disturbance activities related to earthmoving operations, to construction (including, but not limited to, construction of buildings and other structures), and to paving activities, shall be conducted only in conformance with the following principles:
(1) 
During the period of earth moving and, in the case of subdivisions or land developments, prior to the completion of roads, parking areas, buildings and other improvements and the establishment of vegetation or soil stabilization measures, facilities shall be installed to ensure the following:
(a) 
There shall be no discharge of sediment or other solid materials from the site as a result of stormwater runoff.
(b) 
During the period of earthmoving, peak discharges and discharge volumes from the site shall comply with subsection 20-312.B(3), with the following exceptions and additions:
1) 
Any person conducting a business or regular earthmoving (quarrying, top soil removal, etc., but not agricultural operations) shall calculate runoff for facility design based upon runoff before earthmoving and runoff during the maximum period of exposure.
2) 
Wherever soils, topography or other conditions suggest substantial erosion potential during subdivision and land development or earthmoving, the township, as recommended by the township engineer, may require that the entire volume of a two-year storm be retained on site or that special sediment trapping facilities be installed.
(2) 
No earthmoving or stripping of vegetation shall be conducted in areas greater than 25% slope unless specific approval is obtained from the board of supervisors of the township with recommendations from the township engineer.
(3) 
Earthmoving and the addition of fill will be minimized to preserve the natural features and topography.
(4) 
Stripping of vegetation, regrading or other development shall be done in such a way that will minimize erosion.
(5) 
To the maximum extent practical, natural vegetation shall not be removed except as provided in the approved final subdivision or land development plan or the zoning permit. The stockpiling of soil over the roots of trees to be preserved is prohibited within the drip line of the tree.
(6) 
The amount of disturbed area and the duration of exposure shall be kept to a practical minimum.
(7) 
The permanent (final) vegetative and structural erosion control and drainage measures shall be installed as soon as practical.
(8) 
Sediment in runoff water shall be trapped and removed through means approved by the township engineer to assure adequate capacity in the basins or traps.
(9) 
Procedures for protecting soils or geologic formations with water supply potential from contamination by surface water or other disruption by construction activity shall be established.
(10) 
Such other requirements or exceptions as are consistent with these terms in the foregoing principles may be imposed or allowed by the township engineer.
C. 
Special requirements for agricultural and/or logging and woodcutting operations. To prevent accelerated erosion and resulting sedimentation, earth disturbance activities relating to agricultural and/or logging and woodcutting operations should be conducted with consideration to the following principles:
(1) 
Agricultural Operations. All agricultural operations should operate under a conservation plan approved by the Natural Resource Conservation Service (NRCS) and the Chester County Conservation District (CCCD) and conform to the following principles:
(a) 
Permanent cover should be maintained within a minimum of 25 feet of a stream and should be maintained 10 feet up slope from property lines.
(b) 
Permanent cover shall not be disturbed within the township road right-of-way.
(c) 
Tillage and nursery operations shall not be conducted on slopes exceeding 15%, and sod operations shall not be conducted on slopes exceeding 8%, except where minimum tillage methods approved by NRCS or CCCD are followed.
(d) 
Diversion terraces or cover crops no less than 10 feet in width should be provided at a maximum of 200-foot intervals on slopes of 8% to 15% where tillage or sod crops are planted.
(2) 
Logging or Woodcutting Operations. All logging or woodcutting operations conducted with or prior to a land development should be conducted in conformance with the following principles and standards:
(a) 
Stumps, ground cover and root mat must be left intact until land development plans are approved and erosion and sedimentation control facilities are installed.
(b) 
Methods for removal of logs and the removal routes shall be specified in a plan, approved by the township engineer, and shall not include traversing slopes of 15% to 25% at more than the minimum gradient possible.
(c) 
On slopes of 10% to 25%, logging will involve the cutting or removal of less than all of even-aged and non-contiguous trees.
(d) 
On slopes exceeding 25%, logging and woodcutting shall be by specific approval of the board of supervisors of the township and shall be limited to highly selected removal of trees. Maximum precautions shall be taken to avoid destruction or injury of understory brush and trees.
D. 
All earth disturbances greater than 2,000 square feet must be done in conformance with this chapter and under a grading permit issued by the township.
E. 
All earth disturbances greater than one acre must be done under a stormwater NPDES permit and have a post-construction stormwater management plan.
[Ord. No. 223, 4/12/2023]
A. 
Any BMP intended to hold standing water for four days or longer shall be designed to incorporate biologic controls consistent with the West Nile Guidance found in appendix 20-D, PADEP document 363-0300-001 "Design Criteria Wetlands Replacement/Monitoring" (as amended) (or contact the Pennsylvania State Cooperative Wetland Center or the Penn State Cooperative Extension Office for design information).
B. 
Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to safely convey flow up to and including the 100-year proposed conditions with the primary outlet clogged. The height of embankment shall provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year proposed conditions inflow as described above. Should any BMP require a dam safety permit under PA Chapter 105 regulations, the facility shall be designed in accordance with and meet the regulations of PA Chapter 105 concerning dam safety. PA Chapter 105 may require the safe conveyance of storms larger than 100-year event.
C. 
Any drainage conveyance facility and/or channel not governed by PA Chapter 105 regulations shall be designed to convey, without damage to the drainage facility or roadway, runoff from the twenty-five-year storm event. Larger storm events (fifty-year and 100-year storms) shall also be safely conveyed in the direction of natural flow without creating additional damage to any drainage facilities, nearby structures, or roadways. All drainage conveyance facilities and/or channels intended to convey runoff to stormwater management facilities must be designed to convey, without damage to the drainage facility, runoff from the 100-year storm event.
D. 
Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from the facility.
E. 
Roadway crossings or structures located within designated floodplain areas shall be able to convey runoff from a 100-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program—Floodplain Management Requirements.
F. 
Any stormwater management facility located within a PennDOT right-of-way shall comply with PennDOT minimum design standards and permit submission and approval requirements.
G. 
Adequate erosion protection and energy dissipation shall be provided along all open channels and at all points of discharge. Design methods shall be consistent with the Federal Highway Administration Hydraulic Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as amended) and the PADEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008, as amended), or other design guidance acceptable to the township.